Improved subsea riser system

ABSTRACT

An improved riser system which comprises a connector for connecting conduits and a mooring system for mooring the connector to the floor of a body of water. The connector may include a pivoting device. The improved riser system also comprises a buoy system for supporting the connector. The buoy system is configured to provide a fixed buoyancy for the connector, the mooring system and at least a portion of the conduits and for providing variable buoyancy for placement of the connector at a predetermined water depth. The improved riser system may also include a flexible conduit made of titanium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation-in-part of U.S. application Ser. No. 12/785,221, filed May 21, 2010, entitled “IMPROVED SUBSEA RISER SYSTEM,” the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to systems for fluid transportation in deepwater environments. Specifically, the present invention relates to a subsea riser system for the transportation of fluids from, for example, a sea floor to a floating vessel or from the floating vessel to the seafloor.

BACKGROUND OF THE INVENTION

Within various industries, pipes are used to transport fluids from one location to another. In the petroleum industry, for example, pipes are used to transport crude oil and gas from wells on the seafloor to the sea surface, and to a distribution network at least for some distance between the fluid's source and its destination. Proper design of piping systems is important to ensure the transportation of fluids in a safe and environmentally friendly manner. Specifically, a piping system has to be designed so that it maintains its integrity when put in use in its particular application. For example, piping systems for use on land have to be designed to take into account parameters such as the pressure of the fluid being transported, the corrosiveness of the fluid being transported, the environment in which the piping system will be located and seismic activity at the location, to name a few. Designers of piping systems for use in water must contend with such parameters and additional parameters such as hydrostatic pressure (the force exerted by the water due to gravity) and hydrodynamic forces (forces due to the motion of the water).

Hydrostatic and hydrodynamic forces become increasingly more relevant for piping systems as the water depth in which the piping system is installed increases. In the case of offshore petroleum production, pipes, known as risers, extend from the seafloor to sea surface for transporting, for example, oil and gas from a wellhead on the sea floor to a surface facility. Risers in deepwater systems are subjected to significant internal and hydrostatic pressure and hydrodynamic forces. Consequently, designing risers to withstand the internal pressures, hydrostatic pressures and hydrodynamic forces of deep water can be challenging. This challenge is exacerbated when the surface facility to which the riser is connected is a floating platform because movement of the floating platform due to the wave, wind and sea currents can transmit significant stress to the riser. Continuous application of stress to the riser causes fatigue and eventually could rupture the riser.

Close to the surface of a deep body of water, the hydrostatic pressure is low while the hydrodynamic forces are high due to the wind, waves and associated currents. Below the surface currents, there may be submerged currents that cause vortex induced vibrations. For example, in the Gulf of Mexico, the surface currents are typically in the first 200 feet of water depth and the submerged currents can exist in about 1,000 feet of water depth.

In the deeper zones of the water, the hydrostatic pressure is higher and the hydrodynamic forces lower than the zones close to the surface. Taking into account the different forces existing at different depths, one type of riser system includes a flexible conduit in the upper turbulent zone of the body of water. Because the flexible conduit is limited in its ability to withstand hydrostatic pressures and axial tension capacity, the flexible conduit is connected to a catenary riser located in the deeper zone of the water (the catenary riser normally curves gently upward from the sea floor). The catenary riser, often made of steel, is able to withstand the hydrostatic pressures at deeper zones of the body of water. The connection between the flexible conduit and the catenary riser is typically located below that zone in the water where the hydrodynamic forces are high. In some riser systems, a buoy is used to support the catenary riser by attaching the riser to the buoy. However, because the flexible conduit is in the upper zone of water, i.e. the first 200 feet of water depth in the Gulf of Mexico, it moves with the currents and this movement causes stress on the catenary riser because the moving flexible conduit is attached to the catenary riser.

What is more, the demands on riser systems are changing, in part, because drilling is increasingly occurring in deeper and more hostile water depth locations. This development has made it more challenging to provide cost effective riser systems because of the corresponding increase in hydrostatic pressure and hydrodynamic forces as riser systems are deployed in deeper and more hostile water depth locations. An additional challenge in designing current riser systems is a need to accommodate subsea systems that permit the size of gas and oil risers to be on the order of 16 inches in diameter and larger.

As noted above, some current riser systems address the hydrodynamic forces in the turbulent zones close to the surface of a body of water by connecting one end of a flexible conduit to a surface vessel. The other end of the flexible conduit is then connected to a catenary riser made of less flexible material. In order to make the conduit flexible enough to withstand the hydrodynamic forces in the turbulent zone, it comprises several thin layers of steel and elastomeric material (i.e. a composite flexible conduit). The layers of steel and elastomeric material imposes limits on the conduit's bore size and the pressure and temperature it can withstand.

In view of the bore size limitation, it should be appreciated that any change in internal diameter between the catenary riser and one or more flexible conduits connected to the catenary riser makes pigging a complex operation. Pigging involves inserting a device (a pig) into a pipeline and using a fluid to push the pig through the pipeline. As the pig moves through the pipeline, it performs functions such as cleaning the pipeline and, for specialized pigs, inspecting the pipeline. Pigging, in some operations, may need to be done as often as once per week. As the complexity of the pigging operation increases, so does a riser's operational costs.

Though it is possible to pig composite flexible conduits having an internal diameter less than the catenary riser, such an operation adds complexity. For instance, if the catenary riser has an internal diameter of 18 inches and the composite flexible conduit has an internal diameter of 14 inches, current systems provide for a pig that will jump in diameter from 14 inches to 18 inches. It should be noted, however, that pigs usually cannot have a jump in diameter above four inches. What is more, pigs that jump in diameter usually do not work as efficiently as pigs that maintain a constant diameter.

Composite flexible conduits are susceptible to high temperature production fluids. As such, a composite flexible conduit is usually the component that limits a riser system's ability to handle such fluids.

In addition to hydrodynamic forces due to wind, waves and associated currents, described above, the conduits in a riser system are subject to movements caused by a change in the products that pass through the conduits. For example, a flexible conduit and a catenary riser will be installed in saltwater. Subsequently, oil is used to displace the saltwater. In turn, the oil may be later displaced by natural gas. These different fluids have different densities. Thus, as the fluid composition in the conduits changes, the weight of the contents in the conduits and the load on the conduits change. Indeed, it is possible that submerged conduits that initially contained a liquid, which is replaced with a gas, will float up towards the surface of the water. Therefore, as the contents of the different conduits change, the relative loads exerted by the conduits against each other change and cause fatigue of components of the riser system.

The current technology of suspending an SCR directly from a host facility is limited due to motions caused by ultra deepwater host facilities. The use of flexible pipe directly suspended from the host to the seafloor has different limitations due to its own weight, collapse pressure and temperature restrictions. Thus, a need exists to decrease the limitations for fluid conduits extending the entire length between the seafloor and the host. There are many different types of host facilities, each having different associated hull designs and motions. There is a need for a single system solution that has the versatility to adapt for a broad range of hosts facilities including Floating Production Storage and Offloading (FPSO), SPAR, Tension Leg Platforms (TLP), Semisubmersible (SS), Floating Storage and Offloading (FSO) and any other type of floating deepwater facility. In sum, a need exists for an improved riser system that can address the current demands being placed on riser systems.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to an improved riser system and method of installation. Embodiments of the invention reduce the transmission of forces from one portion of the riser system to another through a connector and use a buoy system that provides fixed and variable buoyancy.

One embodiment of the invention includes an improved riser system for use in a deep body of water. The riser system includes two conduits. The first conduit has a first end that is attached to, or is a continuation of, a pipeline located on the sea floor. A connector is connected to the second end of the first conduit. The connector is also connected to a first end of the second conduit and the first and second conduits are coupled together to permit fluid communication between the first and second conduits. The second end of the second conduit is located proximate the surface of the body of water. The connector is configured to reduce transmission of forces from one conduit to the other. The improved riser system includes a mooring system for mooring the connector to the sea floor and a buoy system for supporting the connector, and corresponding portions of the first and second conduits. In embodiments of the invention, the buoy system is attached to the connector and is configured to provide a fixed buoyancy. The buoy system also provides variable buoyancy for adjustment of the buoyancy requirement for the installation method and during the life of the riser. The buoy system is connected to the connector so as to provide vertical support and lateral restraint.

Another embodiment of the invention is a method of installing a riser system in a body of water. The method includes preparing a riser assembly above the surface of the body of water. The preparation of the riser assembly includes connecting a first conduit and a second conduit to a connector so that the first and second conduits are in fluid communication with each other. The preparation of the riser assembly also includes connecting a mooring line to the connector and connecting a buoy system to the connector via a flexible member. When the buoy system is connected to the connector it may be at least partially ballasted. This embodiment of the invention further includes lowering the riser assembly into the body of water to a depth below the surface, and at this point the mooring line is attached to a seabed foundation. While lowering the riser assembly, the first and second conduits can be flooded to provide a slight negative buoyancy, and the mooring line is fixed to the sea floor. After the mooring line is fixed to the sea floor, at least a portion of the buoy system can be deballasted, allowing the connector to stabilize at a second predetermined depth.

In a further embodiment of the invention, the flexible conduit is made of titanium. Due to titanium's strength, low density and elasticity, the flexible conduit may be manufactured out of titanium instead of several layers of steel and elastomeric material. Because of the strength and elasticity of titanium, the wall of a titanium flexible conduit is relatively thin yet strong enough to meet the pressure rating and withstand the hydrodynamic forces required for conduits used in turbulent sections of a body of water.

Further yet, embodiments of the invention involve a two stage installation process of a riser system. The two stage installation process includes assembling two major sections of the riser system above the surface of the water and installing these sections at separate times in the body of water. Each of the major sections includes a buoy apparatus and portion of a connector. The portions of the connector are connected, under the surface of the body of water in which they are deployed, to form the riser system.

Another embodiment of the invention includes a system for pigging a riser. The system includes a pig launching station connected to a first conduit. At least a portion of the first conduit is located on a floor of the body of water. The system also includes a pig receiving station connected to the first conduit. The pig receiving station is configured to receive a pig and liquid displaced from the first conduit by the pig. The first conduit is connected to a second conduit and the second conduit has an internal diameter different from the first conduit.

A further embodiment of the invention includes a system that provides components of a riser system to pivot around a certain point of a connector. For example, embodiments of the invention include a riser system in a body of water having a first conduit with first and second ends. The first end of the first conduit interfaces the seafloor. The riser system also includes a connector that has a pivoting device. The connector is connected to the second end of the first conduit. The riser system also includes a second conduit having first and second ends. The first end of the second conduit is connected to the connector. The first and second conduits are coupled together and are in fluid communication with each other. The riser system also includes a mooring system for mooring the connector to the seafloor. The mooring system includes a tendon connected to the pivoting device. The pivoting device is adapted to allow any one of, or a combination of, the tendon, the first conduit and the second conduit to pivot about the pivoting device when a load is applied to any one of the tendons, the first conduit and the second conduit.

Further yet, embodiments of the invention include a method of installing a riser system. The riser system has a system structure that includes a first conduit connected to a frame, which has a pivoting device. The system structure also includes a buoy having a tubular configuration with a lumen and a lift line passing through the lumen. The method of installing comprises connecting at least one mooring tendon to the frame and deploying the riser system structure in a body of water. The method of installing also includes connecting the at least one mooring tendon to a floor of the body of water. Further, the method includes positioning a second conduit via the lift line passing through the buoy's lumen and connecting the second conduit to the first conduit so that the second and the first conduit are in fluid communication.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It will be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It will also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an illustration of a riser system according to one embodiment of the invention;

FIGS. 2A-2C are illustrations of a connector as it is used in a riser system, according to one embodiment of the invention;

FIGS. 3A and 3B illustrate an installation process for a riser system according to one embodiment of the invention;

FIGS. 4A-4G illustrate an installation process for a riser system according to one embodiment of the invention;

FIG. 5 is an illustration of a riser system according to one embodiment of the invention;

FIGS. 6A-6G illustrate an installation process for a riser system according to one embodiment of the invention;

FIG. 7 is an illustration of a pigging system according to one embodiment of the invention;

FIGS. 8A-8E are illustrations of a connector in a riser system, according to embodiments of the invention;

FIGS. 9A-9D illustrate an installation process for a riser system according to one embodiment of the invention;

FIG. 10 is an illustration of a buoy according to one embodiment of the invention; and

FIG. 11 is an illustration of a buoy according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is an illustration of a riser system according to one embodiment of the invention. Riser system 100 may be for the transportation of oil from a pipeline connected to a wellhead assembly located on seafloor 103 to a floating production, storage and offloading vessel (FPSO) 108. It should be noted that in embodiments of the invention, riser system 100 may be used to transport other types of fluids, such as water and natural gas, and to different types of export and surface facilities, such as a floating LNG facility. Moreover, in addition to the transportation of fluids from the seafloor to a surface facility, riser system 100 may transfer fluids from the surface facility to the seafloor, for example, for production enhancement of a seafloor reservoir.

Referring still to FIG. 1, riser system 100 includes two conduits, steel catenary riser (SCR) 102 and flexible conduit 106. In this configuration, SCR 102 is in fluid communication with a pipeline 109 on seafloor 103 that in turn connects to wellhead assembly 110. Further, SCR 102 can be coupled to flexible conduit 106 at connector 104 so that SCR 102 and flexible conduit 106 are in fluid communication. Thus, fluid from the wellhead assembly 110 may flow through pipeline 109, SCR 102, flexible conduit 106 to FPSO 108. SCR 102 and pipeline 109 are able to withstand the hydrostatic pressures in the deeper portions of body of water 101 and may be made of material such as carbon steel and other alloys, the like and combinations thereof.

Flexible conduit 106 is able to withstand the hydrodynamic forces of the upper levels of body of water 101 and, in embodiments of the invention, is designed to be flexible. Flexible conduit 106 may be made of materials such as steel, alloys and synthetic material the like and combinations thereof.

Connector 104, in embodiments of the invention, is configured to reduce the transmission of forces emanating from the movement of flexible conduit 106 to SCR 102. As such, connector 104 can reduce the overall stress and strain to which SCR 102 is exposed over time.

Connector 104 is preferably moored to seafloor 103 by mooring line 105 and a fastening device 112. Fastening device 112 comprises a suction pile, gravity weight, the like or combinations thereof. Mooring line 105 comprises a synthetic fiber tendon. Mooring lines must be able to accommodate high loads. Consequently, mooring lines have traditionally been made from materials such as wire ropes and chains. Over time, however, the development of synthetic fibers has brought about the use of mooring lines made from synthetic tendons. These synthetic fiber tendons have the advantage of being lighter than wire ropes and chains but able to accommodate as high loads as wire ropes and chains do. Therefore, the use of synthetic fiber tendons as mooring lines allows the riser system as a whole to be lighter than when other mooring equipment is used, particularly in the deeper water of current production activity. Mooring line 105 may comprise materials such as Polyester, Aramid (aromatic polyamid), LCAP (Liquid Crystal Aromatic Polyester), the like, and combinations thereof.

Riser system 100 includes buoy system 107 for vertically supporting the submerged weight of connector 104, mooring line 105, flexible conduit 106 and SCR 102. Buoy system 107 can include a variable buoyancy buoy. As such, buoy system 107 may be unitary or may comprise two or more buoys. Accordingly, buoy system 107 may include fixed buoyancy buoy 107A and variable buoyancy buoy 107B. For example, in one embodiment of the invention, variable buoyancy buoy 107B may be positioned at a fixed depth of about 150-200 feet below the surface of body of water 101 and the fixed buoyancy buoy 107A may be positioned at a fixed depth below variable buoyancy buoy 107B. Because buoy system 107 may be capable of providing variable buoyancy, buoy system 107 facilitates the placement of connector 104 at a desired water depth for the attachment of mooring line 105 to fastening device 112, which fastens mooring line 105 to seafloor 103. Additionally, buoy system 107, when connected to connector 104, is preferably configured to provide only vertical support and thereby lateral restraint to connector 104, mooring line 105, flexible conduit 106 and SCR 102. Therefore, by reducing the transmission of forces from flexible conduit 106 to SCR 102, and providing preferably vertical support only to connector 104 by buoy system 107, the service life of SCR 102 may be improved. Connector 104 is preferably connected to flexible member 111 which is connected to fixed buoyancy buoy 107A. In this manner, connector 104 is suspended from buoy system 107. Thus, flexible member 111 provides the vertical support for connector 104 and to some extent laterally restrains connector 104. However, preferably flexible member 111 does not transfer forces from buoy system 107 to SCR 102 and flexible conduit 106 through the connector 104.

Referring to FIG. 2A, the process for connecting flexible conduit 106 to SCR 102 includes first connecting flexible conduit 106 to connector 104 above the surface of the water. In this embodiment of the invention, flexible conduit 106 is connected to connector 104 by placing flexible conduit 106 on curved support 204. Fastener 205, which may be a circular or loop shape, is used to secure flexible conduit 106 at one end of curved surface 204. Fastener 205 prevents flexible conduit 106 from being dislodged from curved surface 204 but has a large enough diameter to allow flexible conduit 106 to be pulled along curved support 204, as will be described below. After flexible conduit 106 is secured on curved surface 204, connector 104 may be placed in the water. While connector 104 is underwater, SCR 102 may then be pulled into frame assembly 201 using pull lines. After SCR 102 is pulled into frame assembly 201, SCR 102 is locked into frame assembly 201 with a latch mechanism (not shown but well known to those skilled-in-the-art). At this point, there may be gap 203 between SCR 102 and flexible conduit 106.

Referring now to FIG. 2B, to close gap 203 and provide fluid communication between SCR 102 and flexible conduit 106, flexible conduit 106 is pulled down onto SCR 102. Mechanisms known in the art, such as pull lines and hydraulic systems may be used to pull or push flexible conduit 106 onto SCR 102. Once gap 203 is closed, flexible conduit 106 and SCR 102 may be coupled together. In embodiments of the invention, flexible conduit 106 and SCR 102 may be coupled together by a coupling that comprises a Retlock® connector or other such coupling well known to those skilled-in-the-art. In embodiments of the invention, the coupling may comprise the pull-in mechanism for pulling flexible conduit 106 onto SCR 102, (in direction x as shown in FIG. 2B). Further, it should be noted that in a variation of the invention, flexible conduit 106 may first be locked to frame assembly 201, then SCR 102 may be pulled up to and coupled to flexible conduit 106. SCR 102 may then be secured to frame assembly 201. The top portion of frame assembly 201 can be connected to flexible member 111 while the bottom portion of frame assembly 201 is attached to mooring line 105. Referring now to FIG. 2C, this diagram shows how curved support 204 keeps flexible conduit 106 in a bent configuration. Because flexible conduit 106 is in a bent configuration, a force applied to flexible conduit 106 in direction “y,” for example, would bend flexible conduit 106 upwards and pull it away from curved support 204 but not transmit that force to SCR 102. Conversely, a force in the opposite direction of “y” may bend flexible conduit 106 against curved surface 204 but not transmit that force to SCR 102.

FIGS. 3A and 3B illustrate an installation process for riser system 100 according to one embodiment of the invention. FIG. 3A illustrates an aspect of the installation process that may occur above the surface of the water, for example, on installation vessel 301. To begin the process, mooring line 105 may be cast from installation vessel 301 into body of water 101. Mooring line 105 may be connected to connector 104. Additionally, SCR 102 and flexible conduit 106 may be connected to connector 104 so that SCR 102 and flexible conduit 106 are in fluid communication with each other.

Referring now to FIG. 3B, fixed buoyancy buoy 107A may be connected to connector 104 by flexible member 111. Variable buoyancy buoy 107B may be connected to fixed buoyancy buoy 107A, via flexible member 107C, to form buoy system 107. Fixed buoyancy buoy 107A may be a syntactic foam buoy. Variable buoyancy buoy 107B may be a buoyancy tank in, and from, which water may be pumped to vary its buoyancy. Once fixed buoyancy buoy 107A is connected to connector 104, connector 104, SCR 102, flexible conduit 106 and the remaining portion of mooring line 105 are lowered into body of water 101. Because mooring line 105 comprises synthetic fiber, which is relatively light, fixed buoyancy buoy 107A is able to handle loads in deeper zones of body of water 101, as compared to riser systems that use heavier mooring equipment. SCR 102 and flexible conduit 106 are flooded for the riser system 100 to achieve negative buoyancy when buoy system 107 is placed in body of water 101. It should be noted that although buoy system 107 is shown as having two buoys, in embodiments of the invention, buoy system 107 may comprise more than two buoys.

Referring still to FIG. 3B, in embodiments of the invention, fixed buoyancy buoy 107A is designed to partially support the weight of mooring line 105, connector 104, SCR 102 and flexible conduit 106. In this scenario, the riser system 100 is negative buoyant. To continue the installation operation, flexible member 107C and variable buoyancy buoy 107B may be deployed and allowed to sink to a predetermined depth in body of water 101. Variable buoyancy buoy 107B may be deployed in a fully ballasted or partially ballasted mode so that riser system 100 as a whole still has a negative buoyancy. That is, riser system 100 continues to sink but may be supported from a crane located on installation vessel 301. As riser system 100 sinks, seafloor 103 can support more of the submerged weight of riser system 100 as more of SCR 102 rests on seafloor 103.

When the top of variable buoyancy buoy 107B reaches a desired depth, mooring line 105 may be connected to fastening device 112 which in turn may be fastened to seafloor 103. The connection of mooring line 105 to fastening device 112 may be done with the assistance of a Remote Operated Vehicle (ROV). Indeed, any of the operations disclosed herein, in particular those that take place below the surface of body of water 101, may be done with the assistance of an ROV. In embodiments of the invention, once variable buoyancy buoy 107B is at the desired depth and mooring line 105 is connected to fastening device 112, variable buoyancy buoy 107B is deballasted until it exerts an upward force large enough to counteract the weight of riser system 100 and thereby suspend riser system 100 in body of water 101 at a fixed depth. At this point in this embodiment of the invention, riser system 100 is installed and variable buoyancy buoy 107B is positioned vertically above fixed buoyancy buoy 107A so that buoy system 107 provides only vertical support and lateral restraint to connector 104, mooring line 105, flexible conduit 106 and SCR 102.

Typically, riser systems are installed by laying a pipeline from an end location, such as a wellhead, to the SCR location with the end of the pipeline furthest from the wellhead forming the SCR. The SCR is usually located proximate to the expected location of the FPSO. However, in instances where the FPSO is already moored at its final location, it may be desirable to install the riser system so that the installation process begins at the riser location and proceeds towards the wellhead. Referring now to FIGS. 4A-4G, embodiments of the invention that may implement this variation of the installation process may include partially assembling the riser system, which may comprise connecting the pipeline to a connector. The preparation of the partial riser assembly includes connecting a mooring line to the connector with a gravity weight suspended from the connector. A buoy system is then connected to a connector via a flexible member, as discussed above in relation to FIGS. 2A-2B. The gravity weight 112 A may then be placed on the seabed onto a suction pile. The magnitude of buoyancy provided by the buoy system may be sufficient to accommodate the weight of the pipeline/SCR when the pipeline/SCR installation begins.

Referring to FIG. 4A, some embodiments of the invention may include a mooring line that comprises synthetic fiber tendons. Mooring lines made from synthetic fiber tendons usually stretch up to 30% of their original length when a load is applied. If a mooring line stretches after it is installed in a riser system, that stretching may change the whole configuration of the riser system. To prevent this problem, installation of a riser system that includes mooring lines made from synthetic fiber tendons preferably includes stretching mooring line 105 prior to installing it. The stretching process may begin by attaching gravity weight 112A to lowering line 113 and then lowering gravity weight 112A into the water with the lowering line 113. In other words, lowering line 113 may be used to provide support to, and suspend, gravity weight 112A in body of water 101. One end of mooring line 105 is attached to gravity weight 112A prior to placing gravity weight 112A in body of water 101 and the other end secured to installation vessel 301. By increasing the length of lowering line 113 so that it is longer than mooring line 105 (assuming both lowering line 113 and mooring line 105 are suspended from installation vessel 301 at the same level), the load of gravity weight 112A may be transferred from lowering line 113 to mooring line 105. This transference of load to mooring line 105 may stretch mooring line 105 to a desired length. If the desired length is not at first achieved, the process may be repeated to achieve the desired stretching of mooring line 105.

Referring now to FIG. 4B, after stretching mooring line 105, lowering line 113 is detached from gravity weight 112A and flexible conduit 106, buoy system 107 and mooring line 105 are connected to connector 104 above the surface of the water and then lowered into the water. It should be noted that though buoy system 107 is shown as including fixed buoyancy buoy 107A and variable buoyancy buoy 107B, buoy system 107 could be a composite buoy, as discussed further below.

Referring now to FIG. 4C, the installation of the riser system may include the use of a ramp on installation vessel 301 to assemble pipeline 302 and thus installation vessel 301 may act as a pipe laying vessel. Pipeline 302 may be connected to the riser assembly after the riser assembly has been immersed in body of water 101. In embodiments of the invention, the riser assembly includes a fastening device 112, which may be gravity weight 112A. However, in some situations, for example, when seafloor 103 is sloped, it may be necessary to add suction pile 112B, which provides gravity weight 112A with horizontal stability. In this variation of the invention, gravity weight 112A is lowered onto suction pile 112B. Such devices 112 are well known to those skilled-in-the-art.

Referring still to FIG. 4C, after the riser assembly has been placed in body of water 101 at a final predetermined depth, pipeline 302 is payed out into body of water 101. As one end of pipeline 302 reaches the vicinity of connector 104, pipeline 302 is connected to pull lines 303, which in turn runs through connector 104 to winches located on vessel 305. Referring now to FIG. 4D, pull lines 303 can be adjusted in length in order for pipeline 302 to conform to a curvature consistent with a permissible stress level in pipeline 302. Referring now to FIG. 4E, if pull lines 303 are reduced in length by the winches on vessel 305, the end of pipeline 302 will move upward and give pipeline 302 more of a curved configuration.

Referring still to FIG. 4E, installation vessel 301 may continue assembling and paying out pipeline 302 while vessel 305 continues to shorten pull line 303 and thereby pull pipeline 302 towards connector 104. Pull line 303 pulls pipeline 302 into connector 104 and then pipeline 302 is locked onto frame assembly 201 of connector 104. Then pipeline 302 may be connected to flexible conduit 106, similar to the procedure discussed with respect to FIGS. 2A-2B. After pipeline 302 is locked into connector 104 and connected to flexible conduit 106, pull lines 303 may be disconnected from vessel 305 and connector 104. As discussed above with respect to FIGS. 2A-2B, pipeline 302 may be connected to flexible conduit 106 using a coupling suitable for the purpose and this coupling may comprise a Retlock® connector, which is well known to those skilled-in-the-art. As pipeline 302 is payed out with one of its end locked onto connector 104, pipeline 302 sinks and bends into a catenary configuration.

Referring back to FIG. 1, riser system 100, especially one where the FPSO is moored prior to installation of pipeline 302, may require that the section of pipeline 302 extending from connector 104 touches down or intersects with seafloor 103 at a particular point—a desired touchdown point. To illustrate, this concept, the touchdown point is labeled T.P. in FIG. 1 and the desired touchdown point is labeled DTP in FIGS. 4E and 4F. In embodiments of the invention, a preferred method of achieving the DTP is to use connecting lines 306 and 307 to establish the touchdown point. Connecting lines 306 and 307 may be made from wire rope. Referring to FIGS. 4E-4F, connecting line 306 may be attached to pipeline 302 and connecting line 307 may extend from and run through channel 309 in fastening device 112.

Referring now to FIG. 4F, as pipeline 302 approaches seafloor 103, connecting line 306 may be joined to one end of connecting line 307 using an ROV. The other end of connecting line 307 may then be pulled through fastening device 112 up to vessel 305. Connecting line 307, in this configuration, may be used as a hauling line by vessel 305 to ensure that the DTP of pipeline 302 is achieved. Specifically, vessel 305 may apply a pulling force on connecting line 307 in one direction. Connecting line 307 may have a stopper 308, which is too large to go through channel 309. The configuration of connecting lines 306 and 307 (including the position of stopper 308) is such that when connecting lines 306 and 307 are joined and stopper 308 rests against gravity weight 112, the touchdown point will be the intersection of line 306 with pipeline 302. In other words, the distance of line 306/307 from stopper 308 to the end of line 306/307 that intersects with pipeline 302 determines the desired touchdown point.

Referring now to FIG. 4G, pipeline 302 may be installed at one end location, for example, to wellhead assembly 110, and connecting line 306 and 307 may be severed. In its installed position, pipeline 302 comprises SCR 302A and sea floor pipeline 302B. In this configuration, pipeline 302B lies on seafloor 103 and provide fluid communication between wellhead assembly 110 and SCR 302A, which in turn is in fluid communication with flexible conduit 106.

The installed parameters of riser system 100 may vary depending on the body of water in which it is installed and the depth of that body of water. For example, in the Gulf of Mexico, riser system 100 may be installed so that fixed buoyancy buoy 107A is located below submerged currents which typically means greater than 1,000 feet below the surface. Concurrently, the variable buoyancy buoy 107B is located below upper currents and turbulent wave action which typically is about 200 feet below the surface.

The installation methods described above with respect to FIGS. 3A and 3B include performing significant portions of the installation process on an installation vessel. For example, FIGS. 3A and 3B show that connector 104, flexible conduit 106 and SCR 102 are connected together on vessel 301 and then deployed in a body of water. This type of installation can be complex and requires concurrent operation of different types of equipment on vessel 301. Major challenges for installers of riser systems in this type of operation include (1) concurrently managing major aspects of the installation process in limited space on an installation vessel; (2) meeting the time limits set for the installation process; and (3) reducing safety hazards on the installation vessel.

To understand these challenges, it should be noted that some installation processes require at least three different reels on the installation vessel. A first reel is used to hold tendon 105. The length of tendon 105 needed depends on the depth of the water. A second reel is required for holding flexible pipe. A riser system installation typically requires between several hundred feet to 2,000 feet of flexible pipe. A third reel is required to hold the SCR 102/pipe 109. In the installation processes described in FIGS. 3A and 3B, tendon 105, flexible conduit 106, connector 104, buoy 107 and SCR 102 are deployed at the same time, which is demanding on the installation crew and equipment.

Referring now to FIGS. 6A-6G, a two stage installation process is shown that involves the consecutive installation of two major parts of the riser system. The first stage begins on vessel 615 with the assembling of foundation system 600 a. The assembling process includes attaching connector portion 604 a to a mooring system that will be used to moor the riser system to the seafloor 603. The mooring system includes tendon 605, which extends from connector portion 604 a to fastening device 612. The assembling process also includes connecting buoys 613 to connector portion 604 a. Once foundation system 600 a is assembled, it is deployed in water body 601, as shown in FIG. 6B. As foundation system 600 a descends in body of water 601, fastening device 612 is used to fasten mooring line 605 to seafloor 603 by plugging fastening device 612 into device 616, as shown in FIG. 6C. Buoys 613 suspends connector portion 604 a in an area in the water where connector portion 604 a will be connected to the other part of the riser system.

Once foundation system 600 a is installed, the second stage of the installation of the riser system begins. Referring to FIG. 6D, the second stage includes assembling riser structure 600 b on installation vessel 615. Assembling riser structure 600 b includes connecting connector portion 604 b to SCR 602. Connector portion 604 b is configured to mate with, and couple to, connector portion 604 a forming connector 604 (shown in FIG. 6G). Assembling riser structure 600 b also includes connecting flexible conduit 606 to SCR 602 and connecting buoy 607 to connector 604 b.

Referring now to FIG. 6E, after assembling riser structure 600 b, it is deployed in the water. As SCR 602/609 descends in the water, flexible conduit 606 is connected to FPSO 608, as shown in FIG. 6F. Further, an ROV may be used to move connector portion 604 b closer to connector portion 604 a. As connector portion 604 b approaches connector portion 604 a, guide cone 614, which is attached to connector portion 604 a, guides element 615 of connector portion 604 b so that connector portions 604 b and 604 a are properly aligned. Once connector portions 604 a and 604 b are properly aligned, they are connected.

Referring now to FIG. 6G, connector portions 604 a and 604 b form connector 604 which functions in a manner similar to connector 104 described above with respect to FIGS. 1 and 2A-2C. The connection of connector portions 604 a and 604 b may be done by various means well known in the art such as welding and mechanical latching. For example, latch sections l₁ and l₃ are configured to mate and couple l₂ and l₄. Thus, latches l₁/l₂ and l₃/l₄ connect portions 604 a and 604 b, which thereby connect foundation system 600 a to riser structure 600 b. Referring still to FIG. 6G, an installed riser system 600 is shown whereby foundation system 600 a and riser structure 600 b are connected.

This two stage installation process has several advantages. First, the two-stage installation process is less complex as the crews install the foundation system and the riser structure at different times.

Second, the two-stage installation process is more easily managed on vessels with limited space, thereby creating a safer working environment. Essentially, the fewer major processes the installation crew has to perform at any one time, the safer the working environment.

Third, the two-stage installation process allows more installation vessels to install riser system 600. Referring to FIGS. 6A-6G, installation vessel 615 will be required to have a lifting capacity sufficient to raise the entire riser system. However, the two stage installation process described herein reduces the maximum load that the installation vessel needs to support at any one time. This reduction in lifting capacity means more installation vessels are suitable.

Fourth, the two-stage installation process requires less space. There has to be enough deck space on a surface vessel to accommodate the activity. In the two-stage method disclosed herein, all the components do not have to be handled at the same time. Thus, the deck space required on the installation vessel is less.

Referring now to FIG. 5, a riser system 500 is shown for the transportation of fluid from a pipeline connected from a wellhead assembly 510 on seafloor 503 to an FPSO 508. Riser system 500 includes two conduits, SCR 502 and flexible conduit 506. In this configuration, SCR 502 is in fluid communication with a pipeline 509 on seafloor 503 that in turn connects to wellhead assembly 510. Further, SCR 502 is coupled to flexible conduit 506 at connector 504 so that SCR 502 and flexible conduit 506 are in fluid communication. SCR 502 and pipeline 509 are able to withstand the hydrostatic pressures in the deeper portions of water 501 and may be made of material such as carbon steel and other alloys, hybrids, composite materials and combinations thereof.

As noted above, typical composite flexible conduits usually have thick walls of steel and elastomeric material. Further, composite flexible conduits have greater limitations in terms of combined pressure, temperature and inner diameter relative to catenary risers to which they are attached. Consequently, riser systems having composite flexible conduits may require a plurality of flexible conduits for a single catenary riser to achieve equivalent flow, require complex and expensive pigging operations and have fluid temperature limitations based on the temperature limitation of the elastomeric material.

To address these issues, the present invention may have flexible conduits made of titanium. Referring still to FIG. 5, flexible conduit 506 may be made of titanium. Since titanium is strong, has low density and is elastic, flexible conduit 506 can be made entirely of titanium as compared with several layers of different material used for composite flexible conduits. Since flexible conduit 506 is made of titanium, it does not have the limitations composite flexible conduits have with respect to internal diameter, temperature and pressure. As such, flexible conduit 506 can be sized to correspond with the design criteria of the SCR 502. For instance, the need to have multiple smaller internal diameter flexible conduits connected to a larger internal diameter catenary riser is eliminated. A catenary riser having substantially the same internal diameter as the flexible conduit means that the catenary riser may be pigged using a pig whose diameter does not need to be changed. Because flexible conduit 506 has the same or substantially the same internal diameter as SCR 502, it is possible to pig both SCR 502 and flexible conduit 506 in one pigging operation.

The use of titanium to make flexible conduit 506 presents further advantages. For instance, with flexible conduit 506 made of titanium, riser system 500 is able to withstand temperatures higher than riser systems that use composite flexible conduits. The temperature limitation of the conduits in a riser system is becoming increasingly significant as the temperatures of produced fluids increase. For example, and in general, composite flexible conduits are not ideal for temperatures above 250° F. (depending on other design parameters this temperature can be significantly less), while a flexible conduit 506 made of titanium can withstand temperatures significantly higher.

Referring still to FIG. 5, connector 504 is preferably moored to seafloor 503 by mooring line 505 and fastening device 512. Fastening device 512 may comprise a suction pile, gravity weight, the like or combinations thereof, all of which are well known to those skilled-in-the-art. Mooring line 505, in some embodiments of the invention, may comprise a synthetic fiber tendon. Riser system 500 includes buoy system 507 for vertically supporting the submerged weight of connector 504, mooring line 505, flexible conduit 506 and SCR 502. Connector 504 is preferably connected to flexible member 511, which is connected to fixed buoyancy buoy system 507. In this manner, connector 504 is suspended from buoy system 507. Thus, flexible member 511 provides the vertical support for connector 504. However, preferably flexible member 511 does not transfer motions (such as vortex induced vibrations) from buoy system 507 to SCR 502 and flexible conduit 506 through the connector 504. Buoy system 507 includes components 507 a -507 c which operate similarly to components 107 a-107 c described above with respect to FIG. 1.

It should be noted that though the titanium flexible conduit 506 has a greater bend radius relative to composite flexible conduits, it is still less than that of a steel pipe. Accordingly, referring to FIGS. 1 and 5, typically distances R5 and D5 of a riser system 500 made of titanium are greater than distances R1 and D1 of a system 100 that includes a composite flexible conduit.

In some embodiments of the invention, flexible conduit 506 is made of both steel and titanium sections. For example, for sections of flexible conduit 506 that have the most curvature or exposure to significant stress, titanium may be used. Sections 506 a and 506 f, for example, may be tapered stress joints and subjected to significant loads due to the movement of FPSO 508. As such, sections 506 a and 506 f may be made of titanium and typically are about 30 feet in length. Similarly, since section 506 d, known as the dip or sag bend, has a higher curvature than other sections, it may be made of titanium. On the other hand, where strength or elasticity is not critical, steel may be used. Thus, for sections 506 b and 506 e, which are relatively straight and are not subject to high stress, steel may be preferable. Another possible reason for using steel is cost. Different sections of titanium and steel may be joined by methods known in the art such as via welding, mechanical flanges and the like.

The problem described above with respect to the pigging of a riser system having a SCR and a flexible conduit of different internal diameters may be solved by other methods. For example, in the case of a gas export riser in which liquid is periodically deposited in its pipelines, it is difficult to send a pig through the catenary riser section of the pipe when the catenary riser and the flexible conduit have different internal diameters. As mentioned above, one solution is to use flexible titanium conduits that have the same diameter as a catenary riser.

One solution to the problem of pigging different sized conduits is to locate a pig launching device either on the connector or on the seabed at the location of the Pipeline End Termination (PLET). Liquids that are to be displaced in pigging operations frequently accumulate in the valleys of the pipeline that rests ion the seabed since the seabed is not flat. As such, pigging need only be carried out on the section of the pipeline that is on the seafloor and not through the catenary riser section. Thus, the problem of pigging through the catenary riser and the flexible conduit having different diameters is avoided.

Referring now to FIG. 7, riser system 700 includes components 701-712 that operate similarly to components 101-112 of riser system 100. FIG. 7, however, shows pipe 709 having curved sections 709 c and 709 e (valleys) and 709 a, 709 b and 709 d (peaks) due to the unevenness of seafloor 703. Liquid condensate will accumulate in sections 709 c and 709 e. To clear the liquid condensate, riser system 700 has a pigging system that includes pig launching stations 713 and pig receiving station 714. Pig launching and receiving stations are known in the art and are available from, for example, RHINO® Process Equipment. When displacing liquid from sections 709 c and 709 e is necessary, a pig is launched from pig launching station 713. The pig is pushed by a fluid through line 709 until it reaches pig receiving station 714 where the pig and the displaced liquid are removed from line 709. In this system, it is not necessary to have the pig traverse different diameters of flexible conduit 706 and pipe 709. It should be appreciated that riser system 700 could be designed so that pig launching station 713 is located on connector 704 and pig receiving station 714 located on seafloor 703, or vice versa.

Referring to FIGS. 8A and 8B, a riser system 800 is shown which includes an SCR 802 in fluid communication with pipeline 809. Pipeline 809 in turn connects to wellhead assembly 810. SCR 802 is coupled to a flexible conduit 806, at connector 804 so that SCR 802 and flexible conduit 806 are in fluid communication. Flexible conduit 806 is also connected to FPSO 808 as shown. Connector 804 functions as a tension frame. At the top end of connector 804 is a pivoting device 804 b.

Referring to FIGS. 8B-8C and 8E, the pivoting device shown includes a hinge. However, other pivoting devices may be used such as a trunnion, as also discussed below. At the bottom end of connector 804 is crosshead 804 a, which is a beam connected to the sides of frame struts 804 c. Crosshead 804 a supports SCR 802. To moor riser system 800, tendon 805 extends from hinge 804 b to fastening device 812. Buoyancy device 807 is connected to connector 804 to provide buoyancy support to riser system 800.

Referring now to FIG. 8B, a perspective view of connector 804 is shown. Flexible members 811 connects hinge 804 b to buoy 807 (See FIG. 8A). Any number of flexible members may be used in embodiments of the invention. For example, it may be desirable to have more than one flexible member 811 as a safety feature. Similarly, the mooring system may include one or more tendons 805. A plurality of tendons may provide more stability and safety benefits.

Referring again to FIG. 8A, angle A is found between frame strut 804 c and flexible conduit 806, angle B between frame strut 804 c and tendon 805, and angle C between tendon 805 and flexible conduit 806. After installation, angles A, B and C tend to change as a result of, for example, loads resulting from a change in the density of the contents of flexible conduit 806 and SCR 802. Replacing the liquid in flexible conduit 806 and SCR 802 with a less dense gas may cause flexible conduit 806 to move in direction “x” and SCR 802 to move in direction “y.” Movements similar to those indicated in directions “x” and “y” in a conventional riser system bend the flexible conduit and the SCR. In contrast, in the present invention of riser system 800, the forces exerted in directions “x” and “y” would cause either, or both of, flexible conduit 806 and SCR 802 to pivot about pivoting device 804 b. In other words, when certain loads are exerted on flexible conduit 806 and SCR 802, riser system 800 utilizes the pivoting device to allow riser system 800 to move to a new state of equilibrium. Thus, riser system 800 is configured to have loads pass through the pivoting device, allowing riser system 800 to adjust automatically to angular variations between components of the riser system without inducing and storing bending loads on the components, such as flexible conduit 806, SCR 802 and tendons 805.

Referring to FIGS. 8C-8E, different configurations for attaching flexible conduit 806 to connector 804 are shown. FIG. 8C shows flexible conduit 806 running above hinge beam 804 b. It should be appreciated that because connector 804 includes a pivoting device, flexible conduit 806 could be a metal pipe. The pivoting device would, at least partially compensate for the inflexibility of the metal pipe when loads are applied.

FIG. 8D shows flexible conduit 806 passing through the center of trunnion 804 d. In this case, pipe 806 a is a part of flexible conduit 806. It should be noted, however, that instead of connecting flexible conduit 806 directly to SCR 802, pipe 806 a may be a different pipe from flexible conduit 806. In this instance, pipe 806 a is used to connect flexible conduit 806 and SCR 802 at point “P.” The benefit of the design shown in FIG. 8D is that loads applied to flexible conduit 806 are transmitted directly to the center of trunnion 804 d.

FIG. 8E shows a design where flexible conduit 806 is connected to bent pipe 813 having “a gooseneck shape.” Bent pipe 813 is in fluid communication with flexible conduit 806 and SCR 802. Bent pipe 813 is supported by triangular plate 814. In this configuration, a load applied to flexible conduit 806, in direction “z,” is transferred by triangular plate 814 to hinge beam 804 b. Likewise, because the pivoting device is connected to buoy 807, upward pull loads from buoy 807 are directed to the pivoting device.

In sum, as illustrated in FIGS. 8A-8E, embodiments of the invention seek to have all the major loads exerted on riser system 800 transmitted through a pivoting device (e.g. hinge beam 804 b or trunnion 804 d). The loads transmitted to hinge beam 804 b or trunnion 804 d will then cause the components, such as flexible conduit 806 and SCR 802 to pivot around hinge beam 804 b or trunnion 804 d.

Referring to FIGS. 9A-9D, an installation method for a riser system having a connector 904 with a hinge beam 904 b is shown. The installation begins with vessel 908 a transporting buoy 907, tendons 905 and SCR 902/pipe 909 to a desired location in body of water 901. On vessel 908 a, buoy 907 and tendons 905 are connected to hinge beam 904 b. Further, SCR 902/pipe 909 is welded to, or otherwise connected, to crosshead 904 a. Once installation vessel 908 a is at the desired location, vessel 908 a deploys buoy 907, connector 904 and SCR 902/pipe 909 in the water, as shown in FIG. 9A. As connector 904 sinks in the water, tendons 905 are fastened to seafloor 903 by inserting plugs 912 a into receiving devices 912 b, well known to those skilled-in-the-art. Vessel 908 a then unravels SCR 902/pipe 909 from a reel and moves away from connector 904. As SCR 902/pipe 909 descends into the water towards seafloor 903, it pulls connector 904 into a vertical alignment as shown in FIG. 9B.

Referring still to FIGS. 9A and 9B, vessel 908 a continues to move away from connector 904 and completes laying SCR 902/pipe 909 onto seafloor 903. The curved portion 902 is the catenary riser and the portion that lays on seafloor 903 is pipe 909. At this point, flexible conduit 906 is not yet connected to connector 904. To make this connection, flexible conduit 906 is lowered into the water from installation vessel 908 b by, for example, wires L1 and L2, as shown in FIG. 9C. Wires L1 and L2 support flexible conduit 906 up to the side of connector 904. Another lift wire L3 is lowered to the center of connector 904 through lumen 907 a of buoy 907. An ROV is then used to take hold of hook 916 located on line L3. The ROV moves hook 916 to aperture 915 at the top of triangular support 913. L3 is then rolled in to pull flexible conduit 906 towards connector 904 as shown in FIG. 9D.

Wires L1 and L2 are then removed from flexible conduit 906 by an ROV, for example. Wire L3 is then used to lower flexible conduit 906 onto SCR 902 for connection. Supporting flexible conduit 906 through lumen 907 a makes it easier to lower flexible conduit 906 on top of SCR 902. It should be appreciated that the installation process shown with respect to FIGS. 9A to 9D uses the flexible conduit configuration described with respect to FIG. 8C. However, one skilled-in-the-art may apply the installation process to different flexible conduit configurations such as flexible conduit configurations shown in FIGS. 8A and 8B.

Referring back to FIG. 1, embodiments of the invention may include a variable buoy buoyancy and a fixed buoy buoyancy as shown with respect to buoy system 107. The design of buoy system 107 with fixed and variable buoyancy buoys, for installation in riser systems, has several advantages. First, this design makes it easier to install riser systems because it facilitates easy lowering of the riser system at a desired depth. Specifically, the ability to vary the buoyancy provides an ability to change the depth of installation. Second, this variable buoy system provides the ability to select preferred weight requirements. In other words, fixed buoyancy buoy 107A may be selected such that it is still at a slight negative buoyancy at the final operating depth (approximately 1,500 feet in the Gulf of Mexico). Third, in the event variable buoyancy buoy 107B looses buoyancy and sinks, fixed buoyancy buoy 107A may still provide a positive vertical load to support riser system 100 after it sinks marginally, at which point it will reach an equilibrium state (remain suspended). Equilibrium is achieved within body of water 101 because as riser system 100 sinks, some of the weight of SCR 102 will be supported on seafloor 103 rather than by buoy system 107. Fourth, the installation process as disclosed may be easily reversible and thereby facilitates repairs that may be performed above the surface of the water. Specifically, the buoyancy applied to the riser system may be varied and thus after installation, the upward force from the buoy system may be increased to allow the riser system to ascend and be easily removed from the water.

Referring to FIG. 10, an embodiment of the invention may include composite buoy 1007. Composite buoy 1007 comprises fixed buoyancy portion 1007A and variable buoyancy portion 1007B. Fixed buoyancy portion 1007A may comprise syntactic foam or other material providing a constant or fixed vertical load. Variable buoyancy portion 1007B may comprise a tank, to and from, which water may be pumped or any other configuration for providing variable buoyancy. The configuration of composite buoy 1007 may be preferred in selected water depths, particularly if it is desirable to locate the sources of the fixed buoyancy and the variable buoyancy below anticipated upper and loop currents.

Referring now to FIG. 11, an embodiment of the invention may include buoy 1107. Buoy 1107 comprises housing 1108. Housing 1108 encloses syntactic foam buoy elements 1107A, which are separate elements and may be separated by voids 1107B. When syntactic buoy 1107 is deployed in water, its buoyancy effect may be increased by passing a gas, such as air, through, for example, pipe 1109. Conversely, buoy 1107's buoyancy may be decreased by releasing gas from housing 1108 through valve 1110.

In embodiments of the invention, mooring line 105 includes several tendons, which may include synthetic fiber tendons. If one or more tendons break, in this configuration, an unbroken tendon could still maintain the installation in the desired location. Referring again to FIG. 1, if all the tendons break, fixed buoyancy buoy 107A will rise to the surface of body of water 101 or rise to a higher level that is below the surface as it moves into an equilibrium state when the weight of the SCR 102 and pipeline 109 increases (because they are less supported by seafloor 103) to a point when the upward force from buoy system 107 equals the downward force from the weight of SCR 102 and pipeline 109. That is, riser system 100 becomes suspended closer to the surface. In sum, failure of the components of riser systems comprising embodiments of the current disclosure will not cause a catastrophic failure of the whole riser system.

Riser systems according to embodiments of the invention may include several combinations of SCR 102, connector 104 and flexible conduit 106. For example, a first combination of SCR 102, connector 104 and flexible conduit 106 may connect a first wellhead assembly to a manifold assembly on FPSO 108. Concurrently, a second combination of SCR 102, connector 104 and flexible conduit 106 may connect a second wellhead assembly to the same manifold assembly on FPSO 108. Other configurations may also include different combinations of SCR 102, connector 104 and flexible conduit 106 running from the same well head to the manifold on FPSO 108. As one skilled in the art would recognize, such combinations are within the scope of the current invention.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

1-27. (canceled)
 28. An improved riser system for use in a body of water, said system comprising: a first conduit having a first and second end, said first end interfacing the floor of the body of water; a connector connected to said second end of said first conduit; a mooring system for mooring said connector to the floor of the body of water; a second conduit having a first and second end, wherein said second conduit's wall comprises titanium, said connector connected to the first end of said second conduit, said first and second conduits being coupled together and in fluid communication with each other; and a buoy system for providing buoyancy support in said riser system.
 29. The system of claim 28 wherein said buoy system is configured to provide a fixed buoyancy force for said connector, said mooring system and at least a portion of said first and second conduits, and for providing a variable buoyancy force for placement of said connector at a predetermined water depth.
 30. The system of claim 28 wherein said second conduit comprises at least one section of titanium and at least one section of steel.
 31. The system of claim 28 wherein said second conduit and said first conduit have the same internal diameter.
 32. The system of claim 28 wherein said second conduit and said first conduit have substantially the same internal diameter such that said first and second conduits may be pigged by a pig designed for changing diameters.
 33. The system of claim 28 wherein said connector comprises a pivoting device.
 34. The system of claim 33 wherein said pivoting device comprises a hinge.
 35. A method of installing a riser system in a body of water, said method comprising the steps of: assembling a foundation system, wherein said assembling comprises: attaching a first portion of a connector to a mooring system above said body of water, said mooring system comprising a fastening device and a tendon; attaching said mooring system to the floor of the body of water, and suspending said connector in said body of water using a buoyancy device; assembling a riser structure above said body of water, wherein said assembling comprises: connecting a first conduit to a second portion of said connector, said second portion of said connector configured to be coupled with said first portion of said connector; connecting a second conduit to said first connector so that said first and second conduits are in fluid communication; and connecting a buoyancy apparatus to said second portion of said connector; after said assembling of said foundation system and said riser structure, deploying said riser structure in said body of water; and connecting said foundation system to said riser structure by connecting said first portion of said connector to said second portion of said connector.
 36. The method of claim 35 further comprising: guiding said second portion of said connector with a guide cone prior to connection of said second portion of said second connector to said first portion of said second connector.
 37. A system for pigging a riser system in a body of water, said system comprising: a pig launching station connected to a first conduit, wherein at least a portion of said first conduit is located on a floor of said body of water; and a pig receiving station connected to said first conduit, said pig receiving station configured to receive a pig and liquid displaced from said first conduit by said pig, wherein said first conduit is connected to a second conduit, said second conduit having an internal diameter different from said first conduit.
 38. The system of claim 37 wherein said pig launching station and said pig receiving station are located on the floor of said body of water.
 39. The system of claim 37 further comprising: a connector that connects said first conduit to said second conduit, wherein said pig launching station is located on said connector and said pig receiving station is located on the floor of said body of water.
 40. The system of claim 37 further comprising: a connector that connects said first conduit to said second conduit, wherein said pig receiving station is located on said connector and said pig launching station is located on the floor of said body of water.
 41. A method for pigging a riser system in a body of water, said method comprising: launching a pig from a pig launching station connected to a first conduit of said riser system, wherein at least a portion of said first conduit is located on the floor of said body of water and said first conduit is connected to a second conduit, said second conduit having an internal diameter different from said first conduit; and receiving, at a pig receiving station, said pig and liquid displaced from said first conduit by said pig, wherein said pig receiving station is connected to said first conduit.
 42. The method of claim 41 wherein said pig launching station and said pig receiving station are located on the floor of said body of water.
 43. The method of claim 41 wherein said pig launching station is located on a connector that connects said first conduit to said second conduit and wherein said pig receiving station is located on the floor of said body of water.
 44. The method of claim 41 wherein said pig receiving station is located on a connector that connects said first conduit to said second conduit and wherein said pig launching station is located on the floor of said body of water.
 45. An improved riser system for use in a body of water, said system comprising: a first conduit having a first and second end, said first end interfacing the floor of the body of water; a connector that comprises a pivoting device, said connector connected to said second end of said first conduit; a second conduit having a first and second end, said second conduit connected to said connector, said connector connected to the first end of said second conduit, said first and second conduits being coupled together and in fluid communication with each other; and a mooring system for mooring said connector to a floor of the body of water, said mooring system comprising a tendon connected to said pivoting device, said pivoting device adapted to allow any one of or a combination of said first conduit and said second conduit to pivot about said pivoting device when a load is applied to any one of said tendon, said first conduit and said second conduit.
 46. The system of claim 45 further comprising: a buoy system for supporting said connector, said buoy system configured to provide a fixed buoyancy force for said connector, said mooring system and at least a portion of said first and second conduits, and for providing a variable buoyancy force for placement of said connector at a predetermined water depth.
 47. The system of claim 45 further comprising: a triangular support connecting said hinge to said second conduit, said triangular support adapted for reducing a bending load on said second conduit.
 48. The system of claim 45 wherein said pivoting device comprises a hinge.
 49. The system of claim 45 wherein said pivoting device comprises a trunnion.
 50. The system of claim 45 wherein said second conduit passes through said trunnion.
 51. The system of claim 45 wherein said second conduit comprises titanium.
 52. A method of installing a riser system, said riser system having a system structure including a first conduit connected to a frame having a pivoting device and a buoy having a tubular configuration with a lumen and a lift line passing through said lumen, said method comprising the steps of: connecting at least one mooring tendon to said frame; deploying said riser system structure in a body of water; connecting the at least one mooring tendon to a floor of said body of water; positioning a second conduit via said lift line passing through said buoy's lumen; and connecting said second conduit to said first conduit so that said second and said first conduit are in fluid communication. 